Generally, a reactor internal structure installed in a reactor pressure vessel 1 is formed of a material having an excellent corrosion resistance and a mechanical strength under a high temperature and a high pressure environment, such as an austenitic stainless steel and a nickel based alloy. However, even a reactor internal structure formed of such a material may suffer from a material deterioration which is caused by a lengthy operation under a high temperature and a high pressure environment and by an irradiation of neutron. In particular, in a portion near a welding part of the reactor internal structure, when a heat is generated upon welding, a material of the portion may be sensitized or a tensile residual stress may be generated so that a stress corrosion cracking may possibly occur. In this case, it is difficult to change the reactor internal structure for another, which poses a serious problem in terms of maintenance and administration.
An overall structure of the reactor internal structure installed in the reactor pressure vessel 1 in a boiling water reactor electric-power plant (hereinafter referred to as “BWR plant”) is described with reference to FIG. 9. A shroud 2 supporting a fuel assembly is disposed inside the reactor pressure vessel 1. A jet pump 3 is disposed on a part (annulus part) between an inner wall of the reactor pressure vessel 1 and the shroud 2. In addition, disposed on a lower part of the reactor pressure vessel 1 is a control-rod drive mechanism housing 4, and disposed on an upper part of the reactor pressure vessel 1 is a core spray pipe 48 (hereinafter referred to as “CS pipe”).
As shown in FIG. 10, the jet pump 3 includes: a riser pipe 8 located on a side where a coolant is taken in; a mixer nozzle 7 located above the riser pipe 8 and connected to an upper end of the riser pipe 8; and an inlet mixer 5 connected to a lower end of the mixer nozzle 7. A diffuser 6 is connected to a lower end of the inlet mixer 5. In order to support the riser pipe 8 on the inner wall of the reactor pressure vessel 1, a riser brace 9 is mounted on the inner wall of the reactor pressure vessel 1.
Various maintenance/repair devices have been proposed for maintaining and repairing such a reactor internal structure.
At first, there is described a case in which an outer surface of the jet pump 3 is maintained and repaired. In this case, a maintenance/repair device is firstly fitted onto a distal end of a cable or a distal end of an articulated operation pole, and the maintenance/repair device is sent into the reaction pressure vessel 1 from the upper part thereof in a hanging manner. Then, the maintenance/repair device is fixed on a reactor internal structure above the jet pump 3 or the riser brace 9. Thereafter, a target region is subjected to a maintaining and repairing operation by the maintenance/repair device. Such a maintenance/repair device is disclosed in JP11-109081A and JP2002-148385A.
Next, there is described a case in which an inner surface of the jet pump 3 is maintained and repaired. In this case, the inlet mixer 5 is firstly removed. Then, the maintenance/repair device is sent into the reaction pressure vessel 1 from the upper part thereof in a hanging manner. Then, the maintenance/repair device is inserted into the jet pump 3. Thereafter, a target region is subjected to a maintaining and repairing operation by the maintenance/repair device. In an alternative method, the maintenance/repair device is inserted into the diffuser 6 from the lower part of the reaction pressure vessel 1, and a target region is subjected to a maintaining and repairing operation. Such a maintenance/repair device is disclosed in JP5-209864A and JP2003-185784A.
Next, there is described a case in which the inner surface of the jet pump 3 is maintained and repaired without removing the inlet mixer 5. In this case, the maintenance/repair device is inserted into a gap formed in an opening of the mixer nozzle 7, and a target region is subjected to a maintaining and repairing operation. Such a maintenance/repair device is disclosed in JP2001-65778A, JP2002-311183A, and JP2004-251894A.
However, the aforementioned maintenance/repair devices are intended to maintain and repair the diffuser 6 which is a part of the jet pump 3, and thus cannot maintain and repair the inside and the outside of the riser pipe 8.
Next, there is described a case in which a wall of the shroud 2 is maintained and repaired. In this case, a maintenance/repair device is inserted into the annulus part from the upper part of the reactor pressure vessel 1. Alternatively, the maintenance/repair device is inserted from the upper part of the reactor pressure vessel 1 through an upper lattice plate 47 disposed above the shroud 2. Thereafter, a target region is subjected to a maintaining and repairing operation by the maintenance/repair device. Such maintenance/repair device are disclosed in JP Patent Nos. 3288924, 3075952, and 3069005, and JP11-174192A. However, the maintenance/repair devices are intended to maintain and repair the wall of the shroud 2, and thus cannot maintain and repair another reactor internal structure in the reactor pressure vessel 1, in particular, a cylindrical structure such as a pipe.
Next, there is described a case in which a bottom part of the reactor pressure vessel 1 is maintained and repaired. Such maintenance/repair devices are disclosed in JP2002-651159A and JP Patent No. 3011583. Although these maintenance/repair devices are advantageous in their small dimensions and free movableness in water, the maintenance/repair devices cannot maintain and repair the outer surface and the inner surface of the jet pump 3, because the space of the annulus part is further narrower.
Almost all the above-described maintenance/repair devices are sent into the reactor pressure vessel 1 from the upper part thereof through a cable or a wire in a hanging manner, and are brought closer to a cylindrical structure such as a pipe installed in the reactor pressure vessel 1 so as to be fixed onto the outer surface of the cylindrical structure. When the maintenance/repair device is fixed onto the inner surface of the cylindrical structure, a plurality of arms of the maintenance/repair device are expanded in a radial direction of the inner surface of the cylindrical structure. Thus, the plurality of arms are pressed onto the inner surface of the cylindrical structure, whereby the maintenance/repair device can be fixed thereon.
When the aforementioned maintenance/repair device is fixed onto the outer surface of the cylindrical structure, since there are various cylindrical structures of different shapes and different dimensions, it is necessary to change the structure or the configuration of the maintenance/repair device in accordance with a target region to be maintained and repaired. Thus, the structure of the maintenance/repair device is complicated, and the size of the maintenance/repair device is further enlarged.